Influenza Activity Level Definitions

Definitions of the weekly influenza activity estimates as reported by state and territorial epidemiologists.

Activity level

ILI activity* / Outbreaks

Laboratory data

No activity

Low

and

No lab confirmed cases**

Sporadic

Not increased

and

Isolated lab-confirmed cases

or

Not increased

and

Lab confirmed outbreak in one institution†

Local

Increased ILI in one region‡; ILI activity in other regions is not increased

and

Recent (within the past three weeks) lab evidence of influenza in region with increased ILI

or

Two or more institutional outbreaks (ILI or lab confirmed) in one region; ILI activity in other regions is not increased

and

Recent (within the past three weeks) lab evidence of influenza in region with the outbreaks; virus activity is no greater than sporadic in other regions

Regional(doesn't apply to states with four or less regions)

Increased ILI in two or greater but less than half of the regions

and

Recent (within the past three weeks) lab confirmed influenza in the affected regions

or

Institutional outbreaks (ILI or lab confirmed) in two or greater but less than half or the regions

and

Recent (within the past three weeks) lab confirmed influenza in the affected regions

Widespread

Increased ILI and/or institutional outbreaks (ILI or lab confirmed) in at least half of the regions

and

Recent (within the past three weeks) lab confirmed influenza in the state

* ILI activity can be assessed using a variety of data sources including sentinel providers, school/workplace absenteeism, and other syndromic surveillance systems that monitor influenza-like illness.

** Lab confirmed case = case confirmed by rapid diagnostic test, antigen detection, culture, or PCR. Care should be given when relying on results of point of care rapid diagnostic test kits during times when influenza is not circulating widely. The sensitivity and specificity of these tests vary and the predicative value positive may be low outside the time of peak influenza activity. Therefore, a state may wish to obtain laboratory confirmation of influenza by testing methods other than point of care rapid tests for reporting the first laboratory confirmed case of influenza of the season.

† Institution includes nursing home, hospital, prison, school, etc.

‡ Region: population under surveillance in a defined geographical subdivision of a state. A region could be comprised of 1 or more counties and would be based on each state's specific circumstances. Depending on the size of the state, the number of regions could range from 2 to approximately 12. The definition of regions would be left to the state but existing state health districts could be used in many states. Allowing states to define regions would avoid somewhat arbitrary county lines and allow states to make divisions that make sense based on geographic population clusters. Focusing on regions larger than counties would also improve the likelihood that data needed for estimating activity would be available.